1. Field of the Invention
This invention relates to ultrasound biomicroscopy, and in particular to the creation of three-dimensional images in real time of biological structures such as the eye.
2. Description of Related Art
The use of ultrasonic devices is proliferating in the medical arts to produce two-dimensional images of organ structures. An ultrasonic beam is pulsed, typically at 8 MHz, in successive directions in a plane and echo pulses from the tissue interfaces are detected, correlated in time and space, and constructed into a two-dimensional image.
To obtain greater resolution in the image, the frequency of the beam is being increased to the range of 50 to 100 MHz, which is particularly useful in the examination of the eye. These devices provide 50 micron resolution while scanning an area which is 5 millimeters square. This field of view is limited because higher frequency beams are attenuated more rapidly than lower frequency beams in the round trip path back to the detector. The transducer(s) providing and detecting the pulse is housed in a hand-held probe which is moved by the examining opthamologist approximately 3 millimeters from the surface of the cornea. An eye cup is placed around the eye and the volume between the cornea and the transducer is filled with a saline solution to couple the ultrasonic energy from the transducer into the eye. Scanning is performed with a real time image update rate of 8 frames per second. This modality has been useful in elucidating the anatomical correlates of a wide variety of disorders, including anterior segment tumors, plateau iris configuration, malignant glaucoma, and pigment dispersion syndrome. Ultrasound biomicroscopy has also been useful in the evaluation of intraocular lens haptic position, surgical complications, and intraocular foreign bodies. The ultrasound biological microscope is particularly useful where the optical examination is precluded by hyphema or corneal opacification.
Ultrasound biomicroscopic scans are two-dimensional planar images which are composed of discrete picture elements (pixels), defined by X and Y coordinates and a luminance value. Limitations in this art include the limited field of each image and the two-dimensional display. For example, where a foreign object enters the eye, the "track" of its entry is not well displayed by a two-dimensional representation. The high acoustic reflectance of foreign objects in the eye also makes visualization of their size, character, and extent more difficult.
A three-dimensional representation would provide information regarding spatial relationships between involved structures. These images are constructed from many two-dimensional images, stacked behind one another to provide an image with X, Y, and Z dimensions. They are composed of volumetric elements (voxels) which are defined by X, Y, and Z coordinates plus luminescence and transparency parameters. The transparency of a voxel describes the extent to which the observer is able to see through that part of the image to visualize underlying image information.
Accordingly, there is an increased need in the art for a clinically relevant, three-dimensional representation of the features of the eye with an increased field of view.